echoplayer: disable 50 MHz SDMMC bus clock

While 50 MHz works for low activity and small amounts of
data, there are frequent CRC errors when handling larger
transfers. Increasing drive strength only makes it worse.
Everything seems stable at 25 MHz though.

Change-Id: I3471c490ab63b2302b21ee2fe601519ee5a40ce5

firmware/target/arm/stm32/echoplayer/sdmmc-echoplayer.c

@@ -46,8 +46,7 @@ static const struct sdmmc_host_config sdmmc_config INITDATA_ATTR = {
     .bus_widths = SDMMC_BUS_WIDTH_1BIT |
                   SDMMC_BUS_WIDTH_4BIT,
     .bus_clocks = SDMMC_BUS_CLOCK_400KHZ |
-                  SDMMC_BUS_CLOCK_25MHZ |
-                  SDMMC_BUS_CLOCK_50MHZ,
+                  SDMMC_BUS_CLOCK_25MHZ,
     .max_nr_blocks = 65535,
     .is_removable = true,
 };